Lectins are defined as proteins which specifically bind carbohydrates of various types, and initial interest was focused on those isolated from plants such as concanavalin A and ricin agglutinin. These lectins, it was found, were useful in protein purification procedures due to the glycosylation state of a number of proteins of interest. Recently, however, interest has focused on a group of lactose-extractable lectins which bind specifically to certain beta-D-galactoside containing moieties and are found in a wide range of mammalian, invertebrate, avian, and even microbial sources. All of the lectins in this class appear to contain subunits of the order of 12-18 kDa and can be readily classified by virtue of a simple diagnostic test--their ability to agglutinate trypsin-treated rabbit red blood cells is specifically inhibited by certain beta-D-galactose-containing moieties. Thus, although the lectins themselves agglutinate trypsinized rabbit erythrocytes, the agglutination can be inhibited by, for example, lactose, thiodigalactoside and other certain beta-D-galactose containing moieties. Other common characteristics include the lack of requirement for metal ions in effecting agglutination, and the requirement for the presence of a reducing agent such as a thiol.
Gitt, M. A., et al. Proc Natl Acad Sci USA (1986) 83:7603-7607 obtained two cDNA clones from screening a human hepatoma cDNA library with an antiserum specific to a human lung lectin. These cDNAs encoded proteins similar to those found by a number of other workers in a variety of tissues. For example, Kasai, K., et al. in Japanese Kokai 60/184020 describe a human placental lectin of approximately 14 kDa; the sequence of this lectin was shown by the same group to be somewhat similar to that isolated from chick tissues (Ohyama, Y., et al. Biochem Biophys Res Commun (1986) 134:51-56). The chick-derived lectin was shown to be similar in structure to that of discoidin I, which is a lectin observed during the developmental state of the cellular slime mold Dicytostelium discoideum.
Caron, M., et al. Biochim Biophys Acta (1987) 925:290-296 describe the purification and characterization of similar lectins from rat and bovine brain tissue. deCabutti, N.E.F., et al. FEBS Letters (1987) 223:330-334 describe a similar lectin from amphibian ovary. The isolation from eel of a similar "electrolectin" had previously been described by Levi, G., et al. J Biol Chem (1981) 256:5735-5740. An additional analogous 14 kDa lectin was produced by cloning and expression of cDNA derived from various murine fibrosarcoma cell lines by Raz, A., et al. Experimental Cell Research (1987) 173:109-116. A rat lung 14 kDa lectin, and the cDNA encoding it were described by Clerch, L. B., et al. Biochemistry (1988) 27:692-699. Joubert, R., et al. Develop Brain Res (1987) 36:146-150 describe the isolation of lectins from rat brain which are capable of agglutinating brain cells. Raz, A., et al. Cancer Research (1981) 41:3642-3647 describe a variety of lectins from neoplastic cells of various mammalian species.
A comparison of homologies between several animal lectins including the chick, eel, human placenta, human lung, and two hepatoma-derived lectins (all of these lectins described as referenced above) was set forth by Paroutaud, P., et al. Proc Natl Acad Sci USA (1987) 84:6345-6348. It appears that certain amino acid positions of the proteins, including 23, 32, 35, 37, 43, 45, 70, 71, 76, 90, 91, 102, 103, 105, 109-111, are completely conserved in all species compared. Only one lectin of this series, that derived from chicken, contains an "N-linked" glycosylation site, which, however, is not conjugated to saccharide. No mammalian lectin in this family as yet characterized in the art has an N-linked glycosylation site.
Among the soluble lectins, there appear to be a number of varieties with varying molecular weights and/or carbohydrate specificities. Sparrow, C. P., et al. J Biol Chem (1987) 252:7383-7390 describe three classes of soluble lectins from human lung, one of 14 kDa, one of 22 kDa, and a third of 29 kDa. All of these are specific to beta-D-galactosides. The carbohydrate specificities of the 14 kDa class are for the most part similar, but the larger molecular weight species tend to have different specificities. Other species are also noted as showing more than one soluble beta-D-galactoside-binding lectin, including mouse (Roff, C. F., et al. J Biol Chem (1983) 258:10637-10663); rat (Cerra, R. F., et al. J Biol Chem (1985) 260:10474-10477) and chickens (Beyer, E. C., et al. J Biol Chem (1980) 255:4236-4239). Among the various beta-D-galactoside-specific soluble lectins, receptor specificity is considerably different, and the approximately 14 kDa group appears distinct from the 22 kDa and 29 kDa representatives described by Sparrow, et al. (supra).
The preferred lectins of the present invention are isolated from the human promyelocytic leukemia cell line HL-60 or human placenta tissue. Lectins have been isolated from the HL-60 cell line by others, but they do not correspond to lectins of this class. Paietta, E., et al. Cancer Research (1988) 48:280-287 describe a putatively membrane-bound (not soluble) lectin which recognizes N-acetyl neuramic acid as well as galactose terminating biantennary oligosaccharide structures. The activity is independent of calcium. The apparent molecular weight is 17 kDa. Thus, specificity and solubility status differ abruptly from the lectin protein described herein.